Whether you are still in the process of determine if you want to go solar or already have your system installed, the question of “how much power should I be producing” often comes up.
Part of answering this question is understanding the different measurements used when referring to your PV system. The size of your system is specified in watts- the average size system is between 5000 and 7000 watts, or 5 to 7 kilowatts. The output of your system is measured in kilowatt-hours (kWh). Your utility bill is priced in cents per kWh. With these definitions in hand, you can start estimating the kWh’s your system will produce. If you already have a system installed, your installer has already made these calculations. However, it is a good idea to know how to get this information as you monitor your solar production.
Solar modules have a nameplate rating given by the manufacturer. This is the power they can produce in a perfect laboratory condition.
In real world conditions, solar modules never produce their rated kW. The kW rating is used for system sizing, which includes not only the number and type of modules, but also the inverter size and quantity, the power needed at the location to run the system, and the size and length of the electrical wiring.
When they are actually put on your roof, there are a few factors that will “de-rate” the production. These include dust or pollen that can build up on the panels and block some of the sunlight, cloudy days, and power loss as the direct current produced by the panels is converted into alternating current. At the end of the day, the maximum AC power that a solar system will generate is often between 75% and 80% of the nameplate rating.
It is always best to refer back to the signed contract for the estimated monthly system output. It is the responsibility of the system owner to check system production on a daily basis, and to call for service if there looks to be a problem.